Fuel cells have been proposed as a clean, efficient and environmentally responsible power source for electric vehicles and various other applications. In particular, fuel cells have been identified as a potential alternative for the traditional internal-combustion engine used in automobiles.
A typical fuel cell is known as a proton exchange membrane (PEM) fuel cell. The PEM fuel cell typically includes three basic components: a cathode, an anode and an electrolyte membrane. The electrolyte membrane is sandwiched between the cathode and the anode to form a membrane-electrolyte-assembly (MEA). The fuel cell generally also includes porous conductive materials, known as gas diffusion media (GDM), which distribute gaseous reactants over the surfaces of the electrode layers. The reactants typically include a hydrogen fuel and oxygen. The oxygen can be supplied from air, for example. The hydrogen is delivered to the anode and is converted to protons. The protons travel through the electrolyte to the cathode. The electrons in the anode flow through an external circuit to the cathode, where the electrons join the oxygen and the protons to form water. Individual fuel cells can be stacked together in series to form a fuel cell stack capable of supplying a desired amount of electricity.
The MEA is generally interposed between a pair of electrically conductive bipolar plates to complete the PEM fuel cell. The bipolar plates serve as current collectors for the anode and cathode, and have appropriate flow channels and openings formed therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective electrodes. The flow channels generally define lands therebetween that are in electrical contact with the GDM of the fuel cell. Typically, the bipolar plates also include inlet and outlet headers which, when aligned in a fuel cell stack, form internal supply and exhaust manifolds for directing the fuel cell's gaseous reactants and liquid coolant to and from, respectively, the anodes and cathodes.
As is well understood in the art, it is desirable for the membrane within the fuel cell to have a certain relative humidity. The certain relative humidity maintains an ionic resistance within a range effective for conduction of protons across the membrane. During operation of the fuel cell, water from the MEAs and external humidification may enter the anode and cathode flow channels. Typically, the water is forced along the flow channels by a velocity of a gaseous reactant, the velocity of the gaseous reactant being a primary mechanism for water removal from the flow channels. However, if the velocity is not sufficient, water can accumulate resulting in a phenomenon known as stagnation. Stagnant water can block flow channels and reduce the overall efficiency of the fuel cell. For example, the stagnant water may increase flow resistance in particular channels and divert the gaseous reactants to neighboring channels, thereby starving a local area of the reactants. The accumulation of water can also lead to a higher rate of carbon corrosion of the cathode electrode, and a poorer durability under freezing conditions. A high degree of water accumulation or stagnation can lead to fuel cell failure.
Bipolar plates having an increased hydrophilicity are known to positively affect water management in fuel cells. In particular, it is known to treat the anode and cathode flow channels to increase their hydrophilicity. Suitable hydrophilic coatings are known in the art. A thickness of the hydrophilic coating is typically optimized to meet durability and performance requirements for the fuel cell stack. However, it is known that some hydrophilic coatings tend to increase the ohmic contact resistance between the GDM and the bipolar plates. This may lead to a significant loss in stack performance. As a nonlimiting example, a contact resistance of about 20 mohm-cm2 corresponds to a voltage loss of about 30 mV at an operating fuel cell current density of about 1.5 A cm−2. For a fuel cell vehicle that is 50% efficient, for example, this voltage drop amounts to 2.5% loss in performance and 5% loss in fuel economy.
It is known to mask the bipolar plates during application of the hydrophilic coating. The masking limits the application of the coatings to the flow channels, leaving the lands exposed for contact with the GDM after the mask is removed. The mask is generally removed by washing, peeling or scraping. Additionally, hydrophilic coatings have previously been removed prior to drying with a hard rubber surface, e.g. a squeegee. It is further known to polish the lands after application of the hydrophilic coating to the bipolar plates. However, each of these methods is generally inefficient. The squeegee method may result in an undesired accumulation of the hydrophilic coating in the flow channels. Processing steps such as washing, peeling, scraping, and polishing the active surface, for example, may lead to an undesired distortion of the bipolar plates and an undesirable durability due to tearing or cracking of the hydrophilic coating at an interface between the lands and the flow channels.
There is a need for a fuel cell system and a method that provides a desired thickness of the hydrophilic coating on the bipolar plates which has no substantial impact on the contact resistance between the bipolar plates and the gas diffusion media. Desirably, the fuel cell system and method militates against carbon corrosion and improve the durability and performance of the fuel cell system.